This invention relates generally to direction finding systems utilizing a single antenna and more particularly to direction finding systems utilizing a single monopole/crossed slot antenna.
Antenna systems are currently utilized in many direction finding systems in which an antenna receives a propagating signal and determines the approximate angle of arrival of the signal. Many current direction finding systems utilize an array of antenna elements comprising at least two individual antennas. For example, in a typical phase based direction finding system, a pair of antennas would be positioned a predetermined distance from one another. The antennas receive externally propagating signals and utilize the received signals to determine the origin of the signal based upon the difference in phase received by each of the two antenna elements. While such phase based systems do provide direction finding capabilities, such systems typically require at least two antennas which are separated from one another. The antennas thus occupy more surface area upon the platform on which the antennas are mounted than would a single antenna. Additionally, the platform may not be shaped to allow the pair of antennas to be oriented as desired in order to obtain the most accurate and efficient reception.
An alternative direction finding system is one based upon the amplitude of the received signals. In such an amplitude-based direction finding system, two antennas are typically utilized such as those disclosed by Mayes in U.S. Pat. No. 4,443,802 entitled "Stripline Fed Hybrid Slot Antenna." Antennas such as those disclosed by Mayes produce cardioid-shaped radiation patterns which, as illustrated in FIG. 5 of the Mayes patent, may be directed along one of four directions based upon the combination of inputs to the striplines of the antenna. By utilizing a pair of such antennas with each antenna having their cardioid shaped radiation patterns directed in a direction which is a known angular distance from a fixed reference direction, signals may be received by the two antennas which may be analyzed to determine the angle of arrival of the signals. The difference of the amplitudes of the signals received by the two antennas is determined and, based upon a predetermined calibration curve of angular displacement from the fixed reference direction for various amplitude differences, the angular displacement of the signal's emitter from the reference direction may be determined. Such a direction finding system utilizing an antenna as is hereinbefore described, however, suffers from the limitation of requiring at least two antennas to properly determine the direction of the signal. Additionally, antennas such as those disclosed by Mayes may only be directed in four finite directions as opposed to being steerable over the antenna's circumference of 360.degree.. This limitation restricts the number of positions to which the radiation patterns can be directed.
Therefore, it would be desirable to have a direction finding system based upon a single monopole/crossed slot antenna such that the number of antennas required is reduced and the platform space required to mount the antenna is decreased. Additionally, it would be desirable to have a direction finding system utilizing a monopole/crossed slot antenna which is steerable through the antenna's circumference of 360.degree. and configurable to any desired radiation direction.